6.7 KiB
basic
Work in progress, not finished.
This is a BASIC interpreter which is being written from scratch for Collapse OS. There are many existing z80 implementations around, some of them open source and most of them good and efficient, but because a lot of that code overlaps with code that has already been written for zasm, I believe that it's better to reuse those bits of code.
Integrating an existing BASIC to Collapse OS seemed a bigger challenge than writing from scratch, so here I am, writing from scratch again...
Design goal
The reason for including a BASIC dialect in Collapse OS is to supply some form of system administration swiss knife. zasm, ed and the shell can do theoretically anything, but some tasks (which are difficult to predict) can possibly be overly tedious. One can think, for example, about hardware debugging. Poking and peeking around when not sure what we're looking for can be a lot more effective with the help of variables, conditions and for-loops in an interpreter.
Because the goal is not to provide a foundation for complex programs, I'm planning on intentionally crippling this BASIC dialect for the sake of simplicity.
Glueing
The glue.asm file in this folder represents the minimal basic system. There
are additional modules that can be added that aren't added by default, such
as fs.asm because they require kernel options that might not be available.
To include these modules, you'll need to write your own glue file and to hook
extra commands through BAS_FINDHOOK. Look for examples in tools/emul and
in recipes.
Usage
Upon launch, a prompt is presented, waiting for a command. There are two types of command invocation: direct and numbered.
A direct command is executed immediately. Example: print 42 will print 42
immediately.
A numbered command is added to BASIC's code listing at the specified line
number. For example, 10 print 42 will set line 10 to the string print 42.
Code listing can be printed with list and can be ran with run. The listing
is kept in order of lines. Line number don't need to be sequential. You can
keep leeway in between your lines and then insert a line with a middle number
later.
Some commands take arguments. Those are given by typing a whitespace after the command name and then the argument. Additional arguments are given the same way, by typing a whitespace.
Numbers, expressions and variables
Numbers are stored in memory as 16-bit integers (little endian) and numbers
being represented by BASIC are expressed as signed integers, in decimal form.
Line numbers, however, are expressed and treated as unsigned integers: You can,
if you want, put something on line "-1", but it will be the equivalent of line
65535. When expressing number literals, you can do so either in multiple forms.
See "Number literals" in apps/README.md for details.
Expressions are accepted wherever a number is expected. For example,
print 2+3 will print 5. See "Expressions" in apps/README.md.
Inside a if command, "truth" expressions are accepted (=, <, >, <=,
>=). A thruth expression that doesn't contain a truth operator evaluates the
number as-is: zero if false, nonzero is true.
There are 26 one-letter variables in BASIC which can be assigned a 16-bit
integer to them. You assign a value to a variable with =. For example,
a=42+4 will assign 46 to a (case insensitive). Those variables can then
be used in expressions. For example, print a-6 will print 40. All variables
are initialized to zero on launch.
Commands
There are two types of commands: normal and direct-only. The latter can only be invoked in direct mode, not through a code listing.
bye: Direct-only. Quits BASIC
list: Direct-only. Prints all lines in the code listing, prefixing them with their associated line number.
run: Direct-only. Runs code from the listing, starting with the first one.
If goto was previously called in direct mode, we start from that line instead.
clear: Direct-only. Clears the current code listing.
print: Prints the result of the specified expression, then CR/LF. Can be
given multiple arguments. In that case, all arguments are printed separately
with a space in between. For example, print 12 13 prints 12 13<cr><lf>
Unlike anywhere else, the print command can take a string inside a double
quote. That string will be printed as-is. For example, print "foo" 40+2 will
print foo 42.
goto: Make the next line to be executed the line number specified as an
argument. Errors out if line doesn't exist. Argument can be an expression. If
invoked in direct mode, run must be called to actually run the line (followed
by the next, and so on).
if: If specified condition is true, execute the rest of the line. Otherwise,
do nothing. For example, if 2>1 print 12 prints 12 and if 2<1 print 12
does nothing. The argument for this command is a "thruth expression".
input: Prompts the user for a numerical value and puts that value in the
specified variable. The prompted value is evaluated as an expression and then
stored where specified. For example, input x stores the result of the
evaluation in variable x. Before the variable name, a quoted string literal
can be specified. In that case, that string will be printed as-is just before
the prompt.
peek/deek: Put the value at specified memory address into specified
variable. peek is for a single byte, deek is for a word (little endian). For
example, peek 42 a puts the byte value contained in memory address 0x002a
into variable a. deek 42 a does the same as peek, but also puts the value
of 0x002b into a's MSB.
poke/doke: Put the value of specified expression into specified memory
address. For example, poke 42 0x102+0x40 puts 0x42 in memory address
0x2a (MSB is ignored) and doke 42 0x102+0x40 does the same as poke, but also
puts 0x01 in memory address 0x2b.
in: Same thing as peek, but for a I/O port. in 42 a generates an input
I/O on port 42 and stores the byte result in a.
out: Same thing as poke, but for a I/O port. out 42 1+2 generates an
output I/O on port 42 with value 3.
sleep: Sleep a number of "units" specified by the supplied expression. A "unit" depends on the CPU clock speed. At 4MHz, it is roughly 8 microseconds.
Optional modules
As explained in "glueing" section abolve, this folder contains optional modules. Here's the documentation for them.
fs
fs.asm provides those commands:
fls: prints the list of files contained in the active filesystem.
ldbas: loads the content of the file specified in the argument (as an unquoted filename) and replace the current code listing with this contents. Any line not starting with a number is ignored (not an error).